1. Field of the Invention
The invention relates to a rotary position measuring system in accordance with the interferential operating principle.
2. Discussion of Related Art
A so-called interferential three-grating transducer is known, for example, from EP 0 163 362 B1, or from the publication “Längen in der Ultrapräzisionstechnik messen” [Length Measuring in Ultra-Precision Technology] of A. Spiess in “Feinwerktechnik & Messtechnik 98” [Precision Mechanics & Measuring Technology], (1990), 10, pp. 406 to 410. It is possible by means of such a three-grating transducer to generate incremental signals, which are modulated as a function of displacement, by means of which the relative positions of two objects, which can be moved with respect to each other, can be determined. Here, one of the objects, which can be moved with respect to each other, is connected with a scanning unit, which, inter alia, comprises a scanning graduation structure. The other object, however, is connected with a measuring graduation structure, which is scanned by the scanning unit for the generation of the position-dependent signals. While a transparent phase grating is used as the scanning graduation structure, a reflection phase grating is provided as the measuring graduation structure.
Such interferential three-grating transducers, which are operated in incident light, usually have their so-called neutral pivot on the plane of the scanning graduating structure. In this connection, neutral pivot is understood to mean the point around which either the measuring graduation structure or the optical scanning device inclusive of the scanning graduation structure can be pivoted within a defined tolerance range without the output position signal changing its position. However, it is basically desired to move the neutral pivot into an area in which possibly occurring oscillations or tilts of the respective structure during the measuring operation have the least possible effect on the resultant measurement accuracy. In the case of a rotary position measuring system based on such an interferential operating principle in particular, it would be desirable for the neutral pivot to lie on the plane of the rotating graduation structure, or respectively graduated disk, since the graduated disk basically is subject to tilting, tumbling movements, etc. because of bearing tolerances.